Reproduction+Notes+09

IB Biology Notes – Human Reproduction

The information in this document is meant to cover the IB syllabus for topics 6.6 and 11.4 (HL OL!". The Male and Female Reproductive Systems

Images from http://www.dannynicholson.co.uk/learningzone/ody/organs/organs!.htm

Reproductive Hormones

Reproducti"e hormones are in"ol"ed in the changes of puerty# puerty# and in regulation of the menstrual cycle. In females# 

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$ollicle %timulating Hormone &$%H' and (uteinizing Hormone &(H' are produced y the anterior pituitary gland. )strogen and progesterone are produced y the o"ary. o"ary. $%H# (H# estrogen * progesterone work together to control the menstrual cycle.  $%H stimulates maturation of an o"um within its follicle.  +s the follicle grows# it egins producing estrogen# which inhiits $%H# ut stimulates stimulates (H product production. ion. )strogen )strogen also increases increases the thicknes thickness s of the endometrium. stimul ulat ates es o"ula o"ulati tion on and and form format atio ion n of the the corp corpus us lute luteum um## whic which h  (H stim produces progesterone.  ,rogesterone &-the pregnancy hormone' maintains the endometrium and inhi inhii its ts prod produc ucti tion on of $%H $%H * (H# (H# ther there ey y inhi inhii iti ting ng o"ul o"ulat atio ion. n. If fert fertil iliz izat atio ion n does does not not occu occurr# the the corp corpus us luteu luteum m reak reaks s down down## and the the pituitary gland will start producing $%H again to stimulate another follicle. +t this point# the endometrium is epelled from the uterus &menstruation'. )strogen also plays a role in puerty: it promotes the de"elopment of the female secondary seual characteristics.

A. De Jon g/TFS S 2009


Image from http://sciencelogs.com/pharyngula/upload/0112/13/cycle4lg.5pg

In males#  

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$%H promotes spermatogenesis &production of male gametes'. (H &in males# called interstitial cell stimulating hormone or I6%H' stimulates secretion of testosterone. 7estosterone and other hormones collecti"ely known as androgens are produced y the (eydig cells &near lood "essels of the interstitial tissue of the testes'. It stimulates pre8natal de"elopment of the male genitalia# de"elopment of secondary se characteristics &during puerty' and maintenance of the se dri"e. 7estosterone also stimulates maturation of sperm from secondary spermatocytes.



Gonads & Gametogenesis

7he se organs# or gonads# are responsile for production of gametes# or gametogenesis.

Image from http://www.io.miami.edu/dana/!13/gametogenesis.5pg



In females# the o"aries produce o"a # or eggs# y the process of oogenesis# a specialized form of meiosis which only results in formation of one haploid cell: • • • •

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oogenesis egins in utero# efore a female ay is e"en orn a large numer of oogonia are formed y mitosis oogonia undergo cell growth# enlarging an d forming primary oocytes primary oocytes egin meiosis ut stop in prophase I# remaining dormant until puerty egins at onset of puerty# some follicles will egin to de"elop# in response to $%H# and the primary oocyte inside will complete the 9rst meiotic di"ision two cells of uneual sizes are formed – cytoplasm and organelles are gi"en to one cell &now called a secondary oocyte'; the second is essentially a nucleus in a cell memrane < this structure is called a polar ody the polar ody e"entually disintegrates and is reasored y the ody the secondary oocyte proceeds to meiosis II# stopping in prophase II meiosis II completes if the cell is fertilized: this forms an o"um and a second polar ody

In males# the testes produce sperm# y the process of spermatogenesis# also a form of meiosis; howe"er each round of spermatogenesis produces four spermatids# or immature sperm. Ovary Structure

Image from http://www.tarleton.edu/=anatomy/o"ary.5pg



Image from http://www.proteinatlas.org/histology.php>content4id?01

Testis Structure

Image from http://www.we8ooks.com/e(irary/@edicine/,hysiology/Reproducti"e/testis.5pg



Image from http://www.proteinatlas.org/tissue4dictionary/testish!.5pg

%perm are released from the ody within semen# or seminal Auid. It is produced y se"eral glands: %eminal esicles – –

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two pouches located in the pel"ic region ehind the ladder supply a "iscous# nutrient8rich &citric acid# amino acids# fructose' alkaline secretion that forms a part of the seminal Auid supply aout C1D of the "olume of seminal Auid

,rostate Eland –

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largest of all male reproducti"e glands &aout the size of a chestnut'; located 5ust eneath the ladder contriutes aout 21D of the seminal Auid it secretes a thin# milky8white alkaline Auid similar to that of the seminal "esicles; neutralizes acidic Auids oth in the male urethra and the female "agina so that the sperm are not damaged

Bulourethral Eland &6owperFs Eland' – –

pea8sized gland located eneath the prostate gland contriutes aout !1D of the seminal Auid

)pididymis –

tightly8coiled tue located on top of the testis &2 m long if stretched out'

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sperm are stored in the epididymis for aout 0 weeks while they mature# de"elop motility and ecome capale of fertilization



Image from http://faculty.southwest.tn.edu/rurkett/repro4!G.5pg Image from http://distance.stcc.edu/+and,/+,/+,0pages/reprod/spermato.htm

Comparing Spermatogenesis and Oogenesis

%imilarities: ●

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Both spermatogenesis * oogenesis in"ol"e mitosis and cell growth efore undergoing meiosis. 7he hormones (H and $%H are in"ol"ed in regulation of oth processes.

ierences: ● ●

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%permatogenesis occurs in the testes# while oogenesis occurs in the o"aries. %permatogenesis starts at puerty# while oogenesis starts during gestation &fetal de"elopment'. %permatogenesis occurs until death# while oogenesis ceases at menopause. %permatogenesis occurs continuously# whole oogenesis is cyclic &menstrual cycle'. @illions of sperm are created daily# while only one o"um &or two' is produced e"ery 0J days. )5aculation of sperm can occur at any time# while o"ulation only occurs once per menstrual cycle. %perm are produced y eual di"isions# producing four spermatids# while uneual di"isions in oogenesis result in the production of only one o"um. %permatogenesis doesnFt produce polar odies# while oogenesis produces 0 or C polar odies. %permatogenesis reuires testosterone# while oogenesis does not.

Pregnancy

6opulation is the physical contact etween the male and female reproducti"e structures that is needed for sperm to mo"e from the male to the female ut does not necessarily result in fertilization due to the use of a contracepti"e# infertility# or timing. A. De Jon g/TFS S 2009


$ertilization is the fusion of male and female gametes# forming a zygote. ●

7he acrosome reaction is when the sperm comes into contact with the coats surrounding the egg# and as the contact takes place the acrosome &an enzyme8 9lled "acuole at the tip of the sperm' ursts open and releases enzymes that digest the 5elly coat of the egg.

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7he sperm memrane then comes in contact with the egg cell memrane and the two memranes fuse# followed y the entry of the sperm nucleus into the cytoplasm of the egg.

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7he cortical reaction results as the sperm comes in contact with the egg cell memrane# triggering changes to the egg cell memrane making it impossile for other sperm cells to enter.

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+fter fertilization# the chromosomes of the male and female gametes line up at the cellFs euator in preparation for mitosis. %ince this di"ision is not followed y cell growth# and simply clea"es the cell into two smaller ones# it is called a clea"age di"ision. %e"eral clea"age di"isions occur# resulting in the formation of a solid all of cells# the morula# which in humans reaches the uterus aout 3 days after fertilization. Kneual di"isions in the morula occur# resulting in the formation of a Auid89lled space in the middle – it is now a hollow all of cells called a lastocyst. 7he lastocyst implants in the endometrium aout L days after conception. uring 9rst 0 to 3 weeks of de"elopment the emryo otains nutrients directly from the endometrium. Howe"er# tissues grow out from the de"eloping emryo and mingle with the endometrium and form the placenta.

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@aterials are echanged etween the maternal and fetal lood in the placenta. 

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7he placenta is a fetal tissue which in"ades the maternal tissue &the uterus'. It is disk8shaped and grows to aout the size of a dinner plate weighing less than ! kg. iusion of material etween the maternal and emryonic circulatory systems pro"ides nutrients# echanges respiratory gases# and disposes of metaolic wastes for the emryo. Blood from the emryo tra"els to the placenta through arteries of the umilical cord and returns "ia the umilical "ein# passing through the li"er of the emryo.

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In addition# the placenta plays an endocrine role in maintaining pregnancy. In early pregnancy# it secretes human chorionic gonadotopin. +fter the reakdown of the corpus luteum# the placenta secretes oth estrogen and progesterone. If the le"els of these hormones are not maintained# the pregnancy will aort spontaneously ecause the uterus has not maintained fa"ourale conditions for fetal growth.

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H6E acts like pituitary (H to maintain secretion of progesterone and estrogens y the corpus luteum through the 9rst trimester. In the asence of this hormonal o"erride# the decline in maternal (H due to inhiition of the pituitary y progesterone would result in menstruation and spontaneous aortion of the



emryo. (e"els of H6E in the maternal lood are so high that some is ecreted in the urine# where it can e detected in pregnancy tests. ●

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$or the net two months# the growing ay is called an emryo# and is arely discernile as human. In the third month of pregnancy# the growing ay starts to look more human# and is known as a fetus until its irth. 7he fetus is supported and protected y the amniotic sac and amniotic Auid. 

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7he fetus Aoats in the amniotic Auid. 7he Auid is constantly circulated y the fetus swallowing and MinhalingM it# and MehalingM and urinating into it. +mniotic Auid pro"ides cushioning protection to the fetus. It allows for freedom of fetal mo"ement# which permits symmetrical musculoskeletal de"elopment. It maintains a relati"ely constant temperature surrounding the fetus# which protects it from heat loss. It permits proper lung de"elopment# ecause it is inhaled into * ehaled from the lungs.

Birth

+pproimately CJ weeks after conception# the fetus is ready to lea"e the uterus.  

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Before irth# progesterone le"els will drop. It will send a signal to the etra8emryonic memranes – which leads to the secretion of prostaglandins. ,rostaglandins initiate uterine contractions. 6ontraction of the uterus pushes the ayFs head against the cer"i# causing it to dilate. Ner"e endings in the cer"i and uterus tell the rain that contractions are occurring# and the posterior pituitary gland will secrete oytocin. ytocin# prostaglandins and a positi"e feedack system cause the uterine contractions to ecome stronger and longer in duration. Ohen the cer"i is fully dilated &!1 cm'# stage one of irth# &laour' is complete. (aour can take many hours. %tage two &epulsion' occurs as powerful contractions force the ay out of the uterus. %tage two usually lasts no more than one hour. nce the ay has een orn# the uterus has nothing to push against – this ends the positi"e feedack loop# and contractions diminish. 7he 9nal stage of irth is epulsion of the placenta# caused y a few more uterine contractions.



Fertiliation !"#F$ %ome couples are unale to concei"e due to infertility issues such as locked $allopian tues or a low sperm count. I$ is one common way that couples with fertility issues are ale to ecome pregnant. ● + drug is in5ected once a day for three weeks# to stop the womanPs normal menstrual cycle. ● (arge doses of $%H are in5ected once a day for !18!0 days to stimulate the o"aries to de"elop many follicles. H6E is in5ected C2 hours efore egg collection# to loosen the eggs in their follicles ● and to make them mature. 7he man pro"ides semen y e5aculating into a container. 7he sperm are ● processed to concentrate the healthiest ones. ● 7he eggs are etracted from the follicles using a de"ice inserted through the wall of the "agina. )ach egg is mied with sperm in a ,etri dish# which is kept in an incuator ● o"ernight. 7he ,etri dishes are checked the net day to see if fertilization has occurred. ● 7wo or three emryos are selected and placed# "ia a long plastic tue into the ● uterus. ● +out two weeks after placing the emryos# a pregnancy test is done to see if any emryos ha"e implanted. ● + scan is done aout a month after placing the emryos# to see if the pregnancy is continuing normally. 7he hearteat should e "isile. in vitro

%thical Concerns Regarding "#F Pros ●

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Cons

promotes fertility in childless couples suering due to genetic disease could e reduced if emryos were screened efore transferring to the uterus.

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against religious principles multiple irths are more likely# and carry a greater risk to the mother doctors decide which emryos are transferred to the uterus – so humans are deciding who li"es * dies more emryos are produced than are normally implanted in one procedure – spares may e destroyed if they are not stored for later procedures

Reproduction+Notes+09

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